The so-called Long Term Evolution (LTE) wireless communication networks developed by members of the 3rd-Generation Partnership Project (3GPP) use orthogonal frequency-division multiplexing (OFDM) in the downlink and Discrete Fourier Transform spread (DFT-spread) OFDM (also referred to as single-carrier frequency-division multiple access, or FDMA) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing/bandwidth as the downlink and the same number of single carrier FDMA (SC-FDMA) symbols in the time domain as OFDM symbols in the downlink.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds, each radio frame consisting of ten equally-sized subframes of 1-millisecond length, as shown in FIG. 2. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 microseconds.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 milliseconds) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 milliseconds) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
The reference symbols shown in FIG. 3 are the cell specific reference symbols (CRS) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
While the development and deployment of LTE networks provides users with greatly increased wireless data rates and has enabled the development of a wide variety of mobile broadband (MBB) services, demand for these services continues to grow. In addition to this increased demand for improved bandwidth and performance, new applications for special-purpose devices, such as machine-to-machine (M2M) devices, continue to be developed. These market forces indicate that a wireless communications technology with improved flexibility is needed, to better match the variety of service requirements for mobile data applications.
There has been a rapid growth in the number of wireless devices and applications in recent years, and this trend is highly likely to continue in the future. This growth signals a need for a new radio access technology (RAT), which may be regarded as a “5G” (5th-generation) wireless technology. One of the key goals of the current plans for 5G is to expand services offered by the network beyond mobile broadband (MBB). New use cases may come with new requirements. At the same time, 5G should also support a very wide frequency range and be very flexible when it comes to deployment options.
With the emergence of new applications with highly varying application needs, i.e., quality-of-service (QoS) parameters and deployment scenarios, a single, inflexible, physical-layer technology is not adequate to achieve the desired performance characteristics. For example, it is clear that some services require a shorter transmission time interval (TTI), compared to LTE, in order to reduce latency. In an OFDM system, shorter TTIs may be realized by changing subcarrier spacing or subcarrier bandwidth. (The terms subcarrier spacing and subcarrier bandwidth are used interchangeably herein.) Other services need support of relaxed synchronization requirements or very high robustness to delay spread—this may be done, in a system operating with cyclic prefix, by extending the cyclic prefix. These are just examples of possible requirements.
It is clear, however, that selecting parameters such as subcarrier spacing and cyclic prefix lengths is a tradeoff between conflicting goals. Thus, a radio access technology, e.g., the next generation, or “5G,” RAT, advantageously provides flexible support for several variants of transmission parameters, commonly called “numerologies.” Such transmission parameters might be symbol duration, which directly relates to subcarrier spacing in an OFDM system and in several other multicarrier modulation systems, number of subcarriers, or cyclic prefix duration.
Furthermore, it is beneficial to be able to simultaneously support several services on the same band. This allows for a dynamic allocation of resources (bandwidth for example) between the different services, and for efficient implementation and deployment.
One possible approach to physical layer design for a next-generation wireless system is geared towards fulfilling a wide range of varying QoS requirements including latency, reliability and throughput. In one possible new physical layer design, the scalability is adapted using different subcarrier spacing. This approach can support mixed-mode operation, which allows different subcarrier spacings to simultaneously coexist within the same frequency band. This technique might be referred to as multi-mode multicarrier modulation or as involving multiple multicarrier modulation schemes; in this context, the terms “multicarrier modulation scheme” and “multicarrier modulation mode” should be regarded as interchangeable.